Thermal ink jet printers are operative for emitting ink droplets as described in U.S. patent application No. 292,841 which was filed by Vaught et al on Aug. 14, 1981, and which is incorporated by reference herein. Such thermal ink jet printers emit single ink droplets onto a page to form printed characters. The utility of prior art thermal ink jet printers has been inhibited because of the difficulty involved in creating a printed gray scale.
One prior art gray scale approach has been to aim multiple ink jets at one page location to allow variations in the darkness of a printed dot. This prior art approach requires a complex electromechanical system to coordinate the multiple jets. Another prior art approach, described in U.S. Pat. No. 4,353,079, is to have a single ink jet emit a series of discrete droplets to form a single dot on the page. This prior art approach has the inherent problem that the speed at which dots may be printed on the page is seriously limited.
In accordance with the illustrated preferred embodiment of the present invention, a thermal ink jet printer is capable of providing a printed gray scale at a high printing speed. A packet of current pulses is applied to the resistor of a single ink jet to cause emission of a packet of droplets. The interval between individual pulses is long enough that bubble collapse may occur after application of each pulse, yet short enough that the droplets do not individually break off from the ink jet orifice. Thus, the individual droplets within the packet remain connected and merge in flight to form a single drop. The drop breaks off from the orifice only after emission of the last droplet in the packet. Since the probability of ink jet malfunction is related to the rate at which break-offs occur, by having only a single break-off for each packet, i.e., drop, the drop emission rate can approach the single droplet emission rate without an increase in the probability of malfunction. In the illustrated preferred embodiment of the present invention, a multi-tone gray scale is possible at drop emission rates which approach the maximum single droplet emission rate of the thermal ink jet printer.
In the situation where a thermal ink jet printer is operated with a resistor which is undersized for a given ink, it is often the case that a single current pulse is insufficient to cause emission of an ink droplet. The illustrated preferred embodiment of the present invention is useful, in this situation, for providing a pulse packet in which the impulses generated by the individual pulses are combined to cause emission of a single ink drop.